Today we delve into a chapter that deals with an element that at first glance seems irrelevant: the power amplifier. And I say irrelevant because we often don’t pay enough attention to this component, a simple box at the end of our gear that performs relatively simple functions. However, despite the simplicity of this tool, it is surprising how many models and variants we can find.

Home amplifiers

A home amplifier must be designed to deliver true fidelity in our listening. This often means that some areas are left empty, or rather neglected, such as protection against prolonged overloads and stability with any type of loudspeaker. These are the qualities we will find better addressed in professional equipment.

It seems logical to think that our amplifiers, whether in our home studio or for domestic use, will not be forced to withstand heavy working stress, that is, driving demanding loudspeakers for long periods of time.

In any case, it is worth knowing how to tell apart the different types of amplification we can turn to when setting up our home studio.

Types of amplifier
Class A amplifiers
Power stages that draw high, continuous current from their power supply, regardless of whether there is an audio signal or not. This type of amplification has the drawback of generating strong, constant heat. However, the output transistors are always at a fixed, unchanging temperature. In general, we can say that this class of amplification is common in audio circuits and high-end home equipment, since it delivers powerful yet "refined" sound quality.

Class B amplifiers
In this second class of amplification, the peculiarity is that no current flows through the transistors if there is no audio signal present. The drive signal itself biases the transistors so that they conduct and, in turn, drive the loudspeakers.

This characteristic makes these types of amplifiers especially suitable for battery-powered equipment, since consumption is closely tied to the input signal level.

Generally speaking, we could say that the amplification quality of this equipment is lower, and it is suited to applications that don’t require too much refinement, such as telephone systems, portable security transmitters, paging systems, etc.

Class A + B amplifiers
Some equipment receives a small constant supply, independent of the inputs, in addition to the supply produced according to the signal. In other words, we will have a minimum constant supply and, in addition, the amplifier will also increase the power it delivers to the loudspeakers according to the input signals it receives.

We could list a long series of amplifier types, designed for very different applications, from radio frequency to a long et cetera.

In any event, we have no intention of cramming on amplifiers, although it doesn’t hurt to outline how they basically work. For this reason, and having sketched out how an amp works, we move on to the features that professional equipment should provide.

What features should a professional amp have?
The simplest amplifiers will offer us only the input and output connectors. So it will be easy to find units with a single channel, which, despite appearances, is very useful in the professional field, because if a fault occurs, only one channel will go down. If, on the other hand, we had used a stereo amplifier, after the fault or failure we would have lost both channels at once. In short, mono amplifiers are advisable for multi-speaker installations, especially in live setups: music, theater, etc.

As we add more of the bells and whistles an amp can offer, we can find knobs that adjust input and/or output levels, level meters, overload indicators, power shutoff switches (when there is a risk of overload), or bridging switches that will merge the two channels of our stereo amplifier into a single, more powerful mono channel.

Another element we should evaluate when "browsing" for our amp is its ventilation. Many of them will have fans which, if they are noisy, can be a real nuisance during our recording sessions.

How do you read the unit’s spec sheet?
Sensitivity
This tells us how many volts of input the unit in question needs to produce its maximum rated output. For example: (150 watts into 8 ohms) Sensitivity 775 mV = 0 dBu means that with a voltage of 775 mV at the input, the amplifier will deliver 150 watts into an 8-ohm load.

Output power
The manufacturer should specify the maximum power the model can deliver into a given load. I think we are all used to glancing at the wattage a unit is capable of delivering. Perhaps I would only add that we should check whether the figure given is per channel or total.

Frequency response
Frequency response defines the limits within which the amplifier responds equally to all frequencies when delivering very low power. To give you an idea, this would be a possible frequency response specification: " 20 Hz - 20 Hz +- 0.5 dB" .

Distortion
Logically, before a product is brought to market, it must undergo a series of tests, including distortion tests. That is, checking under what conditions the amplifier will distort, and then stating it in the spec sheet that we will examine when buying the unit.

Signal-to-noise ratio
This one seems fairly easy to understand. The spec sheet should state the noise we will get in relation to the signal output. In an average amplifier it is not unusual to find signal-to-noise ratios of 100 dB, bearing in mind that the more powerful the amp, the higher the noise can be without our perceiving it as disproportionate.

Impedance
Without getting into the math and simplifying the matter, a power amplifier needs to be an ideal voltage source. Its output voltage must remain substantially stable with different load impedances.

Damping factor
The cones and diaphragms of loudspeakers tend to keep vibrating a little after the drive signal has stopped. The damping factor is a numerical figure that expresses the amplifier’s ability to control those loudspeaker vibrations.

No doubt we could delve almost endlessly into all the questions that understanding an amplifier can raise, going deeper into phase response, coupling, and so on and so forth.

It’s not hard to conclude that we musicians aren’t going to lose sleep over whether our amp has one resistor or forty; the point is that it sounds good, clear and powerful.

Getting practical...
Having loosely laid out the most basic mechanics and characteristics of a power amplifier, information you can expand on, study, etc., we move on to the considerations that come up most often, and always have, among everyone who has set out to start their own home studio.

I don’t think there are predefined rules when it comes to choosing an amplifier. Apart from a few crazy ideas that have occasionally been put to me, such as using a mono bass guitar combo amp as the amplification base for our whole home studio, or similar notions, the subject of amplification is relatively flexible.

I suppose the first thing is to stop and think, to ask ourselves what we want to do, how we like to work, etc. Some people like to work at very moderate levels, either by preference or because they face soundproofing problems (that is, neighbors banging on the wall). Other people, however, find it essential to hear their parts with a certain "kick" or a certain "body."

Well then, once we are clear on our possibilities and/or preferences, it will be time to consider buying the amplifier.

Someone once asked me whether it was possible to use the amp from a hi-fi system, and I must say that, as long as you have chosen your monitors carefully, you could work this way perfectly well. I know people who use somewhat "rougher" power amps but with enough energy to get anyone dancing. There is also the option of buying powered monitors (be careful with monitors), etc.

In my personal opinion, it’s comfortable to work at a medium/low volume with very good definition during our sessions and for most of the work, and finally to move on to more powerful amplification (and loudspeakers) that allow us, in the final stage of our mixes, to hear the track at full blast with its body, rounded low end, etc. That way we will know how our recordings sound under all conditions or, failing that, under as many conditions as possible.

Conclusion
I hope this chapter leaves the way open for anyone who wants to delve into the more technical characteristics of a power amplifier, and that it gives food for thought to those who have been toying with the idea of starting to organize the amplification in their home studio.